1. Field of the Invention
In at least one embodiment, the present invention is related to bipolar plates used in PEM fuel cells.
2. Background Art
Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”), to provide ion transport between the anode and cathode.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Each electrode has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water, which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of non-porous, electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power.
The electrically conductive plates currently used in fuel cells provide a number of opportunities for improving fuel cell performance. For example, these metallic plates typically include a passive oxide film on their surfaces requiring electrically conductive coatings to minimize the contact resistance. Such electrically conductive coatings include gold and polymeric carbon coatings. Typically, these coatings require expensive equipment that adds to the cost of the finished bipolar plate. Moreover, metallic bipolar plates are also subjected to corrosion during operation. Degradation mechanism includes the release of fluoride ions from the polymeric electrolyte. Metal dissolution of the bipolar plates typically results in release of iron, chromium and nickel ions in various oxidation states.
The efficiency of PEM fuel cell is reduced by wet end shunt currents flowing through the coolant. Such shunt currents depend on the ionic conductivity of, and the potential drop across, the aqueous based coolant. A slight change in coolant conductivity can lead to a significant shunt current which can then damage the wet end plate if it is made of materials that are prone to corrosion, such as stainless steels. This shunt current is mainly due to oxygen evolution on the wet end coolant port area and hydrogen evolution on the dry end plate.
For water management, it is desirable for metal bipolar plates to have a low contact angle at the bipolar plate/water border; that is, a contact angle less than 40°. Titanium nitride coatings have been proposed as corrosion-resistant plating for bipolar plates. Although titanium nitride coatings are cost-effective, such coatings do not provide satisfactory protection for the bipolar plate material. Further, titanium nitride coatings develop relatively low water affinity with a contact angle close to 60°
Accordingly, there is a need for improved methodology for lowering the contact resistance at the surfaces of bipolar plates used in fuel cell applications.